Growth hormone (abbreviated hereinafter to GH) is a proteinous hormone synthesized in adenohypophysis and indirectly promotes growth of bone and differentiation of adipocytes and chondrocytes, and its secretion is promoted by growth hormone-releasing hormone (GHRH) and inhibited by somatostatin [J. Kendrew, et al., Eds., The Encyclopedia of Molecular Biology (Blackwell Science Ltd., London, 1994), p. 462]. GH has not only a growth-promoting action but also actions such as promotion of protein synthesis in various tissues, stimulation of transfer of depot fats and elevation of glycogen content in muscles, and a reduction in GH secretion induces dwarfism, while excessive secretion thereof induces gigantism or acromegaly [Iwanami's Dictionary of Biology, fourth edition, edited by Ryuichi Yasugi, et al. (Iwanami Syoten, Tokyo, 1997), p. 757].
Since human GH has been produced by genetic engineering, GH is used not only for treatment of dwarfism [J. O. Jorgensen, Endocr. Rev. 12, 189 (1991)], but also for treatment of other diseases, and its various effects were found [J. O. Jorgensen, et al., Horm. Res. 42, 235 (1994)]. For example, such effects include activation of reconstitution of osteoblasts and bone in the normal [K. Brixen, et al., Miner. Res. 5, 609 (1990)], enhancement of muscular strength and muscular amount in GH-deficient adults [R. C. Cuneo, et al., J. Appl. Physiol. 70, 688 (1991)], improvement of motility in GH-deficient adults [R. C. Cuneo, et al., J. Appl. Physiol. 70, 695 (1991)], remedy of heavy burns in children [D. N. Herndon, et al., Ann. Surg. 212, 424 (1990)], its combined use with gonadotropins in induction of ovulation [R. Homburg, et al., Clin. Endocrinol. (Oxf). 32, 781 (1990)], prevention of metabolic disorder by administration of prednisone [F. F. Horber and M. W. Haymond, J. Clin. Invest. 86, 265 (1990)], promotion of T cell “education” in heavy immune disorder [w. J. Murphy, et al., Proc. Natl. Acad. Sci. U.S.A. 89, 4481 (1992)], and the effect of inhibiting reduction of the body weight of the aged and the effect of enlarging adipose fat tissues and preventing dermal atrophy [D. Rudman, et al, N. Engl. J. Med. 323, 1 (1990)].
Administration of recombinant GH is effective for promotion of growth in children and normalization of defects in metabolism and functions accompanying GH-deficiency in adults, but there are problems that GH has dose-restricting side effects, cannot be orally administered and is expensive [B. A. Lefker, et al., in Growth Hormone Secretagogues in Clinical Practice, B. B. Bercu and R. F. Walker, Eds. (Marcel Dekker, Inc., New York, 1998), pp. 107-108]. Many adult patients suffer from side effects such as arthralgia and a carpal tunnel syndrome considered to be attributable to pool of excess sodium and humor, so that GH administration cannot be continued [E. Corpas, et al., Endocr. Rev. 14, 20 (1993)]. These side effects are correlated with a non-physiological pattern of hormone secretion by GH administration, and in GH administration, the pulsatility of normal GH secretion cannot be imitated (B. A. Lefker, et al., in Growth Hormone Secretagogues in Clinical Practice, B. B. Bercu and R. F. Walker, Eds. (Marcel Dekker, Inc., New York, 1998), pp. 107-108).
The pulsatility of in vivo GH secretion is established basically by interaction between two regulating factors derived from hypothalamus; that is, GHRH and somatostatin act on pituitary gland to regulate GH secretion [G. S. Tannenbaum and N. Ling, Endocrinology 115, 1952 (1984), R. G. Clark and I. C. Robinson, Endocrinology 122, 2675 (1988)]. The normal pattern of GH secretion differs during the day and night, and during the night, a larger amount of GH is released more frequently. The amplitude of GH release pulse is further regulated by feedback by various steroid hormones, neurotransmitters, GH and insulin-like growth factor, by nutritional status, sleep and motility [J. S. Strobl and M. J. Thomas, Pharmacol. Rev. 46, 1 (1994)].
To overcome the side effects caused by GH administration, a large number of compounds having a GH secretion-inducing action were synthesized, and as growth hormone secretagogue (GHS), their structural activity correlation, their pharmacology and clinical applications were extensively studied. First, peptides such as GHRP-6 (Growth Hormone-Releasing hexapeptide) were synthesized and developed as therapeutic agents for treating disorders attributable to deficiency or reduction in GH [C. Y. Bowers, et al., Endocrinology 114, 1537-1545 (1984)]. However, because these peptide compounds could demonstrate their effect through intravenous injection only, non-peptide compounds having low-molecular weight capable of oral administration were developed [R. G. Smith, et al., Science 260, 1640-1643 (1993)], and some of them have advanced to a phase II clinical test [A. A. Patchett, et al., Proc. Natl. Acad. Sci. U.S.A. 92, 7001-7005 (1995)].
A series of information transfer from signal reception of receptor to functional expression is called signal transduction, and the signal transduction system coupled with G protein proceeds in the following mechanism [Iwanami's Dictionary of Biology, fourth edition, ed. by Ryuichi Yasugi, et al., pp. 555-556 (Iwanami Syoten, Tokyo, 1997)]. This G protein coupled system has a receptor with seven transmembrane domains and is divided into a cAMP system for producing cAMP as a second messenger and inositol-1,4,5-triphosphoric acid (IP3) and diacyl glycerol (DG) inositol phospholipid information transduction system. The cAMP activates cAMP-dependent kinase (A kinase), to cause phosphorylation of serine and threonine residues in functional protein to modify its activity. On the other hand, IP3 binds to IP3 receptor on endoplasmic reticulum to promote release of calcium ions, while DG activates C kinase to promote the action of hormones etc.
The mechanism of increasing the intracellular calcium ion concentration in the signal transduction system with IP3 or DG as second messenger [J. Kendrew, et al., Eds., The Encyclopedia of Molecular Biology (Blackwell Science Ltd., London, 1994), p. 136-137] is as follows: When a ligand binds to the receptor, phospholipase C is activated via G protein, to covert PIP2 into IP3. By IP3, calcium ions pooled in endoplasmic reticulum (ER) as intracellular granule are released into cytoplasm, thus increasing calcium ion levels in the cytoplasm. If IP3 or calcium ions are present in the cytoplasm, the calcium is incorporated again into the endoplasmic reticulum, thus lowering calcium ion levels in the cytoplasm. That is, the binding of the ligand to the receptor causes a transient increase in calcium ion levels in the cytoplasm.
Since GHS acts synergistically on the GH secretion and increase of intracellular cAMP levels by GHRH [K. Cheng, et al., Endocrinology 124, 2791-2798 (1989)] and the binding of GHRH to the receptor induces production of cAMP as second messenger while GHS induces an increase in the intracellular calcium ion concentration, it was suggested that the working mechanism of GHS is different from that of GHRH [J. Herrington and B. Hille, Endocrinology 135, 1100-1108 (1994).], and GHS was supposed to bind to a receptor different than GHRH receptor. Actually, a gene for a receptor to which GHS is bound was cloned, and from the result of Northern analysis, it was found that GHS receptor (GHS-R) is expressed in hypothalamus and brain pituitary gland, and that there is 90% or more homology between the amino acid sequences of porcine- and human-derived GHS receptors [A. D. Howard, et al., Science 273, 974-977 (1996)]. However, an endogenous ligand that binds to GHS-R has not been isolated, and this GHS-R was an orphan receptor whose ligand was not evident.
In some cases, fatty acids such as myristic acid, geranic acid, palmitoyl acid or farnesyl acid are bound to the amino-terminal of a certain protein or to side chains of its amino acid residues, and the role of these fatty acids is anchoring such fatty acid-modified protein to cell membrane [J. Kendrew, et al., Eds., The Encyclopedia of Molecular Biology (Blackwell Science Ltd., London, 1994), p. 616]. In such fatty acid-modified protein, the fatty acid binds to a cysteine residue via S-acyl linkage, and neither an amino acid having fatty acid bound to serine residue via O-acyl linkage, such as the endogenous GHS disclosed in the present invention, nor protein or peptide containing such fatty acid-modified amino acid, is known. Neither is it known for which the peptide containing such fatty acid-modified amino acid functions as a ligand for any receptor.